WHY MUST I PAY FOR THEIR MISTAKE???

My assignment was lost by Intec after I couriered it down to them. Now they are trying to blame it on me and insinuate that the parcel was never sent to them, i.e. they are calling me a ****! I have requested a cancellation on my course now because it has been a few weeks and they still cannot find it.They want me to pay 80% of my course value (which I paid in cash UPFRONT before the course even began). This is unbelievable!!!! Why must i be held responsible for their misplacement of my assignment??? The treatment of their students is disgusting, and the fact that they can even insinuate that I am lying about sending the assignment though simply because they lost it is mind boggling!!! They are treating me like an imbecile. I am very very unhappy with Intec at this stage and I want a solution this problem. <br>